The present invention relates to broadband digital radio communication of at least several Mbps. More particularly, the present invention relates to a method and apparatus for setting a transmission condition in a radio communication system which conducts communication between radio communication terminals based on a measure of the line quality of the channels being used.
In a conventional indoor wireless system, it is normal that a transmitted radio signal is substantially reflected and received through a plurality of different paths depending on the condition of the transmission environment (such as the ceiling, floor, or furniture). FIG. 12 is an illustration showing a state of radio wave transmission in a room. Radio waves radiated from a terminal 1210 are received by a terminal 1212 through a plurality of paths. A signal passing through a path 1201 is not reflected but it is directly received (this signal is referred to as xe2x80x9cdirect signalxe2x80x9d). A signal passing through a path 1202 is reflected once and then received and a signal passing through a path 1203 is reflected twice and then received. These signals 1202 and 1203 are each referred to as a xe2x80x9cmultipath signalxe2x80x9d. Because received signals passing through different paths are different from each other in phase, interference occurs between carrier waves and a phenomenon called frequency selective fading occurs in which signal power suddenly decreases at a specific frequency, as shown in FIG. 13.
The influence of this phenomenon remarkably differs between the case of millimeter wave transmission (30 to 300 GHz) and the case of quasi-microwave transmission (e.g., 2.4 GHz). When a carrier wave is a millimeter wave, it greatly attenuates compared to the case of a quasi-microwave even when both waves are transmitted over the same distance. Therefore, in the case of a quasi-microwave, even a multipath signal repeatedly reflected in a space many times keeps a high-enough intensity and causes interference. As a result, distance differences between paths causing interference are widely distributed. In the case of a millimeter wave, however, distance differences between paths causing interference are concentrated on a limited range. As a result, frequency selective fading occurs in the whole frequency band used in the case of a quasi-microwave. In the case of a millimeter wave, however, the phenomenon strongly occurs in part of the frequency band used, as shown in FIG. 13.
Moreover, in the case of broadband communication, the object of the invention, requiring a symbol transmission rate of several mega-symbols/sec or more, the problem of inter-symbol interference (ISI) occurs. This is because a signal is transmitted at a high speed and thereby, delay of a multipath signal cannot be ignored. As shown in FIGS. 14A-14D, the delay time of a multipath signal accounts for a large part of the time corresponding to one symbol in the case of high-speed transmission, and a delay equivalent to most part of the length of one symbol (in the case of the multipath signal 1202) or more (in the case of the multipath signal 1203) occurs. Thereby, symbols Axe2x80x2 and Axe2x80x3 of multipath signals corresponding to symbol A of the direct signal 1201 strongly interfere with symbol B which is the next symbol of the direct signal.
Thus, in the case of high-speed transmission, strong ISI occurs between multipath signals and the direct signal. Attenuation of the multipath signals is small because the path lengths are not very large compared to the case of the direct signal. Therefore, it is impossible to decode a reception signal 1400 on which the above multipath signals are superimposed.
A conventional apparatus for solving the ISI problem due to high-speed transmission is disclosed in the proceedings of Symposium on Development of Frequency Resources 96, pp. 25-35. To moderate the ISI problem producing ghost signals, the conventional apparatus divides a high frequency band into a plurality of narrow frequency bands and limits the symbol transmission rate of each narrow frequency band so that the ISI does not become a problem in each narrow frequency band.
The transmission method of constructing a broadband channel from a plurality of narrow frequency bands is generally known as frequency division multiplex (FDM) method.
In an indoor environment, the line quality changes in a short time because the radio wave transmission environment frequently changes compared to the case of an outdoor environment. Therefore, the line quality becomes very unstable and thus, there is a problem that the error rate may suddenly increase or the line may be cut off even if the FDM method is directly applied.
When high-speed transmission (10 Mbps or more) is used, it is expected that a multimedia radio terminal capable of transmitting data or video in addition to voice, by a presently used wireless terminal (much less than 10 Mbps), can be realized. For this reason, it is necessary to transmit signals having various communication characteristics at the same time. For example, the communication characteristics of voice allows for the error rate to be a little high but the variation in transmission delay cannot be large. On the other hand, the communication characteristics of ordinary data allows for a variation in transmission delay but communication errors can be a problem.
In the conventional apparatus, it has been impossible to provide a line quality corresponding to the quality of service required for information to be transmitted and communications to be performed. Therefore, when the conventional apparatus is applied to multimedia communication a problem arises that communication efficiency is reduced because communication is performed at a line quality higher than required.
Further, while the demand for a multimedia terminal using high-speed communication and a wide frequency band exists, a demand that communication of only voice or data or multimedia communication using a lower quality of service also exists. For an indoor radio LAN, it would be convenient if a high-performance multimedia terminal using a wide frequency band can communicate, in the same system, with a portable terminal of low cost, low power consumption and uses a narrow frequency band.
An object of the present invention is to provide a method and apparatus for setting a transmission condition in a radio communication system which conducts communication between radio terminals based on a measure of the line quality of the channels being used.
The present invention provides a communication method and system of which sets a transmission condition in a broadband radio communication system. The broadband radio communication system conducts communication between first and second radio communication terminals on a plurality of narrowband channels obtained by dividing a broadband channel. The broadband radio-communication system can be a digital system.
The communication method and system of the present invention performs the steps or the functions of setting a transmission condition of at least part of the narrowband channels in the first radio communication terminal, transmitting a wireless communication from the first radio communication terminal to the second radio communication terminal on the at least part of the narrowband channels for which the transmission condition has been set, measuring in the second radio communication terminal a line quality of each narrowband channel of the at least part of the narrowband channels upon receipt of the wireless communication from the first radio communication terminal, and transmitting line quality information indicating the line quality of each narrowband of the at least part of the narrowband channels from the second radio communication terminal to the first radio communication terminal. The setting or function is performed again using a new transmission condition for each narrowband channel of the at least part of the narrowband channels based on the line quality information received from the second radio communication terminal.
The present invention further provides a radio communication terminal which is usable in a broadband radio communication system conducts communication between a plurality of the radio communication terminals on a plurality of narrowband channels obtained by dividing a broadband channel. The radio communication terminal includes a transmitter which transmits a wireless communication on at least a part of the narrowband channels to another radio communication terminal according to a transmission condition which has been set, a receiver which receives wireless communication on the at least a part of the narrowband channel from the other radio communication terminal according to the set condition and a detector which detects a line quality of each narrowband channel of the at least part of the narrowband channels upon receipt of the wireless communication from the other radio communication terminal.
The radio communication terminal further includes a controller which causes the transmitter to transmit line quality information representative of the line quality of each narrowband channel of the at least part of the narrowband channels detected by the detector to the other radio communication terminal and sets a new transmission condition for each narrowband channel of the at least part of the narrowband channels based on the line quality information received from the other radio communication terminal.